A recyclable and reusable K2PtCl4/Xphos-SO3Na/PEG-400/H2O system for highly regio- and stereoselective hydrosilylation of terminal alkynes

Article abstract of DOI:10.1039/c7gc02823g

K₂PtCl₄/Xphos-SO₃Na in a mixture of poly(ethylene glycol) (PEG-400) and water is shown to be a highly regio- and stereoselective catalyst for the hydrosilylation of terminal alkynes with hydrosilanes. The reaction could be conducted under mild conditions, yielding a variety of functionalized β-(E)-vinylsilanes in good to excellent yields with a total β-(E)-selectivity. The isolation of the products is readily performed by extraction with cyclohexane and more importantly, both expensive K₂PtCl₄ and Xphos-SO₃Na in a PEG-400/H₂O system could be easily recycled and reused at least eight times without any loss of catalytic activity.

Full text of DOI:10.1039/c7gc02823g

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Recyclable and reusable K2PtCl4/Xphos-SO3Na/PEG-400/
H2O system for highly regio- and stereoselective
hydrosilylation of terminal alkynes
Caifeng Xu, Bin Huang, Tao Yan and Mingzhong Cai*
Key Laboratory of Functional Small Organic Molecule, Ministry of Education and
College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang
330022, P. R. China
E-mail: mzcai@jxnu.edu.cn
K2PtCl4/XphosꢀSO3Na in a mixture of poly(ethylene glycol) (PEGꢀ400) and water is
shown to be a highly regioꢀ and stereoselective catalyst for the hydrosilylation of
terminal alkynes with hydrosilanes. The reaction could be conducted under mild
conditions, yielding a variety of functionalized βꢀ(E)ꢀvinylsilanes in good to excellent
yields with a total βꢀ(E)ꢀselectivity. The isolation of the products is readily performed
by the extraction with cyclohexane and more importantly, both expensive K2PtCl4 and
XphosꢀSO3Na in PEGꢀ400/H2O system could be easily recycled and reused at least
eight times without any loss of catalytic activity.
Introduction
Vinylsilanes are especially valuable intermediates which can undergo a number of
important chemical transformations such as Hiyama crossꢀcouplings,1 FriedelꢀCrafts
reactions,2 protodesilylation,3 DielsꢀAlder reactions,4 reduction of the double bond,5
coupling with aldehydes,6 and Heckꢀtype couplings,7 etc.. In general, vinylsilanes
show reactivity similar to that of certain organometallic vinyl derivatives, but may
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offer some attractive advantages of low cost, low toxicity, high functionality tolerance
and chemical stability. Transitionꢀmetalꢀcatalyzed hydrosilylation of alkynes provides
a simple and straightforward, atomꢀeconomical method for the preparation of vinylꢀ
silanes.8 Since the first report of catalytic hydrosilylation of alkynes using Speier’s
catalyst,9 a variety of transition metal catalysts have been developed for related transꢀ
formations including Pt,10 Rh,11 Ru,12 Ir,13 Pd,14 Au,15 Co16 and Cu.17 Among these
transitionꢀmetal catalysts, platinum complexes18 and heterogeneous platinum catalysts
19
are by far the most well developed catalyst systems for the hydrosilylation of
alkynes, providing the βꢀisomer, in the Eꢀgeometry as the major product. The main
difficulty with the catalytic hydrosilylation of alkynes is control of the stereoꢀ and
regiochemistry of the vinylsilanes products since three isomeric products, αꢀ, βꢀ(E)ꢀ,
and βꢀ(Z)ꢀisomers, may be formed with terminal alkynes. Generally, the selectivity is
mainly affected by several factors including the nature of the hydrosilane, the alkyne,
the metal species and the ligand, and reaction conditions. Industrial catalysts such as
Speier’s (H2PtCl6) and Karstedt’s catalysts (Pt2(dvtms)3: dvtms = divinyltetramethylꢀ
disiloxane) can produce high catalyst turnovers; however, the regioꢀ and stereoseꢀ
lectivities can be poor with terminal alkynes as substrates.20
The use of bulky trialkylphosphine ligands21 or platinum Nꢀheterocyclic carbene
complexes22 was reported to be an efficient solution to this problem. These sterically
hindered platinum catalysts can impart high levels of selectivity on the reactions.
Recently, a combination of Pt(II) salts and bulky Xphos23 and platinum(0) olefin
complexes of a bulky terphenylphosphine ligand24 also proved to be efficient catalysts
2

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for the hydrosilylation of terminal alkynes with excellent regioꢀ and stereoselecꢀ
tivities. Although these methods reported are highly efficient for construction of βꢀ
(E)ꢀvinylsilanes, the use of a homogeneous catalytic system in organic solvents on an
industrial scale is a challenge because both platinum and bulky phosphine ligands are
highly expensive, cannot be recycled, and difficult to separate from the product. These
problems are of particular economic and environmental concerns in industry. Moreꢀ
over, in contemporary chemistry, the impact and consequence of reaction solvents on
human health and environment needs to be carefully considered. Therefore, from the
standpoint of green and sustainable chemistry,25 the development of a recyclable and
reusable catalyst system that allows for efficient synthesis of βꢀ(E)ꢀvinylsilanes via
the highly regioꢀ and stereoselective hydrosilylation of terminal alkynes in green
solvents is worthwhile.
In order to address both recyclability and environmental concerns, a simple and
efficient strategy is to immobilize the catalyst in a liquid phase by dissolving it into a
nonvolatile and nonmixing liquid, such as ionic liquids26 and poly(ethylene glycols)
(PEGs). However, ionic liquids impart some disadvantages, such as a complicated
procedure for the preparation of ionic liquids as well as their environmental safety,
which is still being debated since the toxicity and environmental burden data are
unknown for the most of them. It is well known that PEGs are readily available and
inexpensive, thermally stable, recoverable, biodegradable and nontoxic compounds
which serve as efficient media for environmentally friendly and safe chemical reacꢀ
tions.27 In recent years, PEGs have been successfully utilized as reaction media for the
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Pdꢀcatalyzed crossꢀcoupling reactions28 and other transition metalꢀcatalyzed organic
transformations29 with facile recyclability of solvents as well as catalysts. However, to
the best of our knowledge, the platinumꢀcatalyzed hydrosilylation of alkynes in PEGs
have not been reported until now. We herein report the application of K2PtCl4/Xphosꢀ
SO3Na/PEGꢀ400/H2O system as an extremely effective and reusable catalytic medium
for the highly selective hydrosilylation of terminal alkynes leading to βꢀ(E)ꢀvinylꢀ
silanes in good to excellent yields with a total βꢀ(E)ꢀselectivity (Scheme 1).
Scheme 1 Platinumꢀcatalyzed highly selective synthesis of βꢀ(E)ꢀvinylsilanes
Results and discussion
Initially, the hydrosilylation of phenylacetylene 1a with triethylsilane 2a was chosen
as a model reaction to determine the optimum conditions and the results are listed in
Table 1. At first, the solvent effect was examined, and a significant solvent effect was
observed. It is evident that the reaction proceeded slowly in H2O at 25 or 60 ºC and
the desired product 3a was isolated in low yield (Table 1, entries 1 and 2). When
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PEGꢀ400 alone was used as solvent, the reaction at 60 ºC gave 3a in only 62% yield
(Table 1, entry 4). Although the conversion rate of phenylacetylene 1a was low, a total
βꢀ(E)ꢀselectivity could be observed with H2O or PEGꢀ400 alone as solvent due to the
presence of XphosꢀSO3Na as the ligand. To our delight, a mixture of PEGꢀ400 and
H2O was found to be more effective than PEGꢀ400 or H2O (Table 1, entries 5ꢀ9). The
reaction run in PEGꢀ400/H2O (2:1) at 60 ºC gave 3a in 94% yield (Table 1, entry 6).
Our next studies focused on the effect of reaction temperature on the model reaction.
Lowering reaction temperature to 25 °C resulted in a significant decrease in yield and
a longer reaction time was needed (Table 1, entry 10). Raising reaction temperature to
80 °C could shorten reaction time, but did not improve the yield (Table 1, entry 11).
Besides, the efficiency of various chain length PEGs on the model reaction was also
examined under the same reaction conditions (Table 1, entries 12ꢀ15). PEGꢀ400 was
found to be superior to PEGꢀ600 and PEGꢀ1000. Finally, increasing the amounts of
K2PtCl4 and XphosꢀSO3Na had no significant improvement in the yield of 3a, whilst
reducing the amounts of K2PtCl4 and XphosꢀSO3Na resulted in a decreased yield and
a longer reaction time was required (Table 1, entries 16 and 17). Thus, the optimal
catalytic system involved the use of K2PtCl4 (1 mol%), XphosꢀSO3Na (2 mol%) in
PEGꢀ400/H2O (2:1) at 60 °C under Ar for 6 h (Table 1, entry 6).
Table 1 Optimization of platinumꢀcatalyzed hydrosilylation of phenylacetylene with
triethylsilane a
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Entry
1
Solvent (v/v)
Temp. (ºC) α/β ratiob Time (h)
Yield (%)c
H2O
25
60
25
60
60
60
60
60
60
25
80
60
60
60
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
0:100
24
12
24
12
7
28
35
37
62
90
94
86
79
71
52
93
58
87
53
84
94
79
2
H2O
3
PEGꢀ400
4
PEGꢀ400
5
PEGꢀ400/H2O (3:1)
PEGꢀ400/H2O (2:1)
PEGꢀ400/H2O (1:1)
PEGꢀ400/H2O (2:3)
PEGꢀ400/H2O (1:2)
PEGꢀ400/H2O (2:1)
PEGꢀ400/H2O (2:1)
PEGꢀ600
6
6
7
7
8
7
9
10
24
4
10
11
12
13
14
15
16d
17e
12
6
PEGꢀ600/H2O (2:1)
PEGꢀ1000
12
6
PEGꢀ1000/H2O (2:1) 60
PEGꢀ400/H2O (2:1)
60
4
PEGꢀ400/H2O (2:1)
60
12
a Reaction conditions: phenylacetylene 1a (1.0 mmol), Et3SiH 2a (1.5 mmol), K2PtCl4 (1.0 mol%),
XphosꢀSO3Na (2.0 mol%), solvent (1.0 mL) under Ar. b Determined by GC analysis and 1H NMR
c
d
in the crude reaction mixture. Isolated yield of the mixture of 3a and 3a'. K2PtCl4 (2.0 mol%)
and XphosꢀSO3Na (4.0 mol%) were used. e K2PtCl4 (0.5 mol%) and XphosꢀSO3Na (1.0 mol%)
were used.
With the optimized reaction conditions in hand, we began to examine the reaction
with a wide range of substrates to determine the specificity and scope of substrates.
Thus, a variety of substituted phenylacetylenes 1b-1k were reacted with triethylsilane
under the optimized conditions, and the results are listed in Table 2. As shown in
Table 2, it is evident that most of the platinumꢀcatalyzed hydrosilylation reactions
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proceeded smoothly under mild conditions in PEGꢀ400/H2O medium, affording the
corresponding βꢀ(E)ꢀvinylsilanes in good to excellent yields with a 100% βꢀ(E)ꢀselecꢀ
tivity. In all cases, no other isomers were observed in the NMR spectra of the crude
products. For example, both electronꢀrich phenylacetylenes 1b-1f and electronꢀdefiꢀ
cient phenylacetylenes 1g-1h gave the desired βꢀ(E)ꢀvinylsilanes 3b-3h in 72ꢀ93%
yields. The results showed that the electronic properties of substituents on benzene
ring have limited influence on the hydrosilylation reaction. Next, we evaluated the
effect of ortho substituent groups on the reaction selectivity. When 2ꢀmethylphenylꢀ
acetylene 1i and 2ꢀmethoxyphenylacetylene 1j were used as substrates, again a total
βꢀregiocontrol was observed and the expected βꢀ(E)ꢀadducts 3i and 3j were produced
in 80% and 92% yields, respectively, which indicating that the regioselectivity of the
H–Si bond addition is governed by steric effects induced by XphosꢀSO3Na ligand
rather than orthoꢀdirecting effect. To support this explanation, the hydrosilylation of
2ꢀmethylphenylacetylene 1i with Et3SiH was performed without XphosꢀSO3Na and
formed a 35:65 ratio of α:β regioisomers. It is noteworthy that bulky 1ꢀethynylꢀ
naphthalene 1l also underwent the hydrosilylation smoothly to afford the desired
product 3l in good yield with a 100% βꢀ(E)ꢀselectivity. Interestingly, although the
reactivity of 2ꢀethynylpyridine 1m, a heteroarylacetylene was lower than that of
phenylacetylenes, the reaction also proceeded effectively at 80 ºC in the presence of
K2PtCl4 (5 mol%), XphosꢀSO3Na (10 mol%) leading to a single βꢀ(E)ꢀvinylsilane 3m
in 68% yield. A conjugated enyne 1n proved to be also suitable substrate and
produced the desired βꢀ(E)ꢀadduct 3n in 78% yield.
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Table 2 Platinumꢀcatalyzed hydrosilylation of terminal alkynes with triethylsilane
in PEGꢀ400/H2O a,b
a
Reaction conditions: terminal alkyne (1.0 mmol), Et3SiH (1.5 mmol), K2PtCl4 (1.0 mol%),
XphosꢀSO3Na (2.0 mol%), PEGꢀ400/H2O (2:1, 1.0 mL) at 60 ºC under Ar for 6 h. b Isolated yield.
c K2PtCl4 (5 mol%) and XphosꢀSO3Na (10 mol%) were used at 80 ºC for 12 h.
After completion of investigation of functionalized terminal arylalkynes, further
examination showed that the K2PtCl4/XphosꢀSO3Na/PEGꢀ400/H2O system efficiently
catalyzes the hydrosilylation of terminal aliphatic alkynes. For instance, the reactions
of hexꢀ1ꢀyne 1o and 5ꢀphenylpentꢀ1ꢀyne 1p with Et3SiH afforded exclusively their
corresponding βꢀ(E)ꢀvinylsilanes 3o and 3p in high yields, respectively. The reaction
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worked equally well with aliphatic terminal alkynes bearing functional groups.
5ꢀChloropentꢀ1ꢀyne 1q and 4ꢀhydroxybutꢀ1ꢀyne 1r could undergo the hydrosilylation
with Et3SiH effectively to furnish the desired βꢀ(E)ꢀadducts 3q and 3r in good yields.
In addition to aromatic and aliphatic terminal alkynes, we were also pleased to
observe that the K2PtCl4/XphosꢀSO3Na/PEGꢀ400/H2O system was still efficient with
propargylic alcohols, propargylic ethers and propargylic esters. For example, the
reactions of propargylic alcohols 1s-1u, regardless of primary, secondary and tertiary
alcohols, produced the corresponding vinylsilanes 3s-3u in 79ꢀ90% yields with a
100% βꢀ(E)ꢀselectivity. Propargylic ethers 1v and 1w could give the desired βꢀ(E)ꢀ
adducts 3v and 3w in good yields. Interestingly, the reaction of propꢀ2ꢀynꢀ1ꢀyl acetate
1x with Et3SiH also gave exclusively the expected βꢀ(E)ꢀvinylsilane 3x in 74% yield.
In all cases, no other isomers could be observed prior to purification.
Encouraged by the above results, we next investigated the reaction of terminal
alkynes with dimethyl(phenyl)silane 2b under the reaction conditions optimized for
triethylsilane 2a and the results are listed in Table 3. We were pleased to find that the
standard conditions were compatible with a variety of terminal alkynes. It was found
that electronꢀneutral, electronꢀrich and electronꢀdeficient aromatic terminal alkynes
underwent the hydrosilylation reaction with dimethyl(phenyl)silane 2b efficiently and
generated the corresponding vinylsilanes 4a-4c in excellent yields with a 100% βꢀ(E)
ꢀselectivity. The reaction of conjugated 1ꢀethynylcyclohexꢀ1ꢀene with 2b also worked
well under the standard reaction conditions to afford the desired βꢀ(E)ꢀadduct 4d in
78% yield. The reactions of aliphatic terminal alkynes such as 6ꢀhydroxyhexꢀ1ꢀyne,
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5ꢀcyanopentꢀ1ꢀyne, and 4ꢀhydroxybutꢀ1ꢀyne with 2b could proceed smoothly to give
the corresponding βꢀ(E)ꢀvinylsilanes 4e-4g in good to excellent yields. In addition,
both propargylic alcohols and propargylic ethers also proved to be good substrates.
For example, the reactions of propargylic alcohols 1s-1u with 2b produced the correꢀ
sponding βꢀ(E)ꢀadducts 4h-4j in 76ꢀ87% yields. The reactions of 2ꢀ(propꢀ2ꢀynꢀ1ꢀ
yloxy)ethanol and 2ꢀ((propꢀ2ꢀynꢀ1ꢀyloxy)methyl)oxirane with 2b could afford the
desired vinylsilanes 4k and 4l in high yields, respectively. Also, in all cases no other
isomers were observed in the NMR spectra of the crude products. A wide range of
functional groups such as methyl, methoxy, chloro, cyano, hydroxy, epoxy and ester
were well tolerated in present catalytic system.
Table 3 Platinumꢀcatalyzed hydrosilylation of terminal alkynes with dimethylꢀ
(phenyl)silane in PEGꢀ400/H2O a,b
a Reaction conditions: terminal alkyne (1.0 mmol), PhMe2SiH (1.5 mmol), K2PtCl4 (1.0 mol%),
XphosꢀSO3Na (2.0 mol%), PEGꢀ400/H2O (2:1, 1.0 mL) at 60 ºC under Ar for 6 h. b Isolated yield.
Table 4 Platinumꢀcatalyzed hydrosilylation of internal alkynes with triethylsilane or
dimethyl(phenyl)silane in PEGꢀ400/H2O a,b
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a Reaction conditions: internal alkyne (1.0 mmol), Et3SiH or PhMe2SiH (1.5 mmol), K2PtCl4 (1.0
b
mol%), XphosꢀSO3Na (2.0 mol%), PEGꢀ400/H2O (2:1, 1.0 mL) at 60 ºC under Ar for 6 h.
Isolated yield. c Regioselectivty was determined by 1H NMR.
We also performed hydrosilylation reaction of internal alkynes with triethylsilane
2a or dimethyl(phenyl)silane 2b under the reaction conditions optimized for terminal
alkynes and the results are summarized in Table 4. Aromatic or aliphatic symmetrical
internal alkynes 5a-c could undergo the cisꢀhydrosilylation with 2a or 2b smoothly to
afford exclusively the corresponding (E)ꢀvinylsilanes 6a-6d in excellent yields. For
unsymmetrical internal alkynes, the hydrosilylation reactions also proceeded highly
stereoselectively to give the expected syn adducts, but the regioselectivity depended
on the nature of the internal alkynes. For example, electronꢀdeficient unsymmetrical
internal alkynes, ethyl 3ꢀphenylpropiolate 5d and ethyl octꢀ2ꢀynoate 5e underwent the
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cisꢀhydrosilylation with 2b effectively to furnish the corresponding (E)ꢀethyl 2ꢀ
(dimethyl(phenyl)silyl)ꢀ3ꢀphenylacrylate 6e and (E)ꢀethyl 2ꢀ(dimethyl(phenyl)silyl)ꢀ
octꢀ2ꢀenoate 6f in high yields with high regioselectivities (93ꢀ95%) due to the strong
electronꢀwithdrawing nature of ester. However, when 1ꢀphenylpropꢀ1ꢀyne 5f was used
as substrate, despite the different steric hindrances between phenyl and methyl, the
hydrosilylation reaction with 2a afforded a poor regioselectivity (6g/6g' = 62/38).
In order to show the consistency of this protocol we moved towards exploration of
this methodology on the gram scale level (Scheme 2). Under the optimized reaction
conditions, 1.02 g (10 mmol) of 1a and 1.16 g (10 mmol) of 1b were taken as starting
substrates and 1.99 g (91%) of 3a and 2.27 g (90%) of 4b were observed respectively.
Recently, Alami and Cook reported a platinumꢀcatalyzed hydrosilylation of terminal
alkynes and propargylic alcohols by using a PtCl2/Xphos catalyst system in THF.23
Although a variety of βꢀ(E)ꢀvinylsilanes were obtained in high yields with excellent
regioꢀ and steroselectivities, expensive platinum catalyst and Xphos ligand could not
be separated from the reaction mixture and recycled, which limiting its applications
on a large scale. To examine the reusability of the solvent, the catalyst as well as the
ligand, the hydrosilylation reaction of phenylacetylene 1a (1.0 mmol) with Et3SiH 2a
(1.5 mmol) was evaluated in the presence of K2PtCl4 (1.0 mol%) and XphosꢀSO3Na
(2.0 mol%) in PEGꢀ400/H2O (2:1, 1.0 mL) at 60 ºC under Ar for 6 h. The observaꢀ
tions of recyclability are demonstrated in Fig. 1, before the light brown color reaction
mixture 1 was observed, while after the completion of the reaction the color changed
to orange 2. With cooling of the reaction mixture at room temperature, 5 mL of
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cyclohexane was added and shaken, the two layers 3 were generated. The upper layer
of cyclohexane contains the product directly used for purification and the lower layer
contains PEG/H2O with a catalytic system. The PEG/H2O layer was heated to 60 ºC in
vacuum for 20 min to remove the residual cyclohexane. The cooled PEG/H2Oꢀ
catalytic system was used for the next cycle by charging with the same substrates
(phenylacetylene 1a and Et3SiH 2a) without the addition of K2PtCl4 and Xphosꢀ
SO3Na. We were pleased to observe that the K2PtCl4/XphosꢀSO3Na/PEGꢀ400/H2O
system could be recycled and reused eight times without any loss of activity. The
results of eight runs within an average reaction time of 7 h showed that they were
almost consistent in yields (94%, 93%, 94%, 94%, 93%, 92%, 93%, and 92%,
respectively) (Fig. 2).
Before reaction
1
PEG/H2O + Catalyst
+ Starting material
Starting
material
4
PEG/H2O +
Catalyst
After reaction
2
PEG/H2O +
Catalyst +
Product
Recycle
Cyclohexane
+ Product
SiR1
3
Addition of
Cyclohexane
R
PEG/H2O +
Catalyst
Product
3
Fig. 1 Procedure for reuse of the catalyst and the solvent system.
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Scheme 2 Gram scale synthesis of (E)ꢀvinylsilanes 3a and 4b.
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Recycling times
Fig. 2 Recycle of the catalytic system.
The hydrosilylation of terminal alkyne 1 with hydrosilane 2 catalyzed by K2PtCl4/
XphosꢀSO3Na catalyst system in PEGꢀ400/H2O may proceed through a mechanism
analogous to that proposed for the PtCl2/Xphos catalyst system in THF (Scheme 3).23
First, the reaction of K2PtCl4 with XphosꢀSO3Na ligand produces the PtCl2L2 (L =
XphosꢀSO3Na) complex, which is then reduced with hydrosilane 2 to the platinum(0)
catalyst, associated with one or two XphosꢀSO3Na ligands. Subsequently, oxidative
addition of hydrosilane 2 to the platinum(0) catalyst generates the platinum(II) comꢀ
plex A or B and a further coordination with terminal alkyne 1 would give intermediate
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C. Then terminal alkyne 1 would insert into the Pt–H bond (ChalkꢀHarrod mechanism)
rather than Pt–Si bond as was suggested by ab initio molecular orbital and Møllerꢀ
Plesset perturbation theory calculations.30 In the presence of XphosꢀSO3Na ligand, the
100% selectivity for the βꢀ(E)ꢀisomer 3 can be rationalized by the formation of the
transition state I as it would be sterically less demanding and constitutes the unique
pathway to give the vinylplatinum(II) complex D, while the transition state II, is more
congested due to steric repulsion between the R substituent and the sterically hindered
XphosꢀSO3Na ligand. Finally, reductive elimination of intermediate D affords the
desired βꢀ(E)ꢀvinylsilane 3 and regenerates the platinum(0) catalyst to complete the
catalytic cycle.
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Scheme 3 Proposed catalytic cycle.
Conclusions
In conclusion, a highly efficient and reusable K2PtCl4/XphosꢀSO3Na/PEGꢀ400/H2O
system for the highly regioꢀ and stereoselective hydrosilylation of terminal alkynes
with hydrosilanes has been developed. In the presence of K2PtCl4 (1.0 mol%) and
XphosꢀSO3Na (2.0 mol%), the hydrosilylation reactions of a variety of functionalized
terminal arylalkynes as well as aliphatic alkynes with hydrosilanes such as Et3SiH and
PhMe2SiH proceeded smoothly and efficiently at 60 °C in a mixture of PEGꢀ400 and
water to afford the desired βꢀ(E)ꢀvinylsilanes in good to excellent yields with a total
βꢀ(E)ꢀselectivity. In addition, under the same reaction conditions symmetrical internal
alkynes underwent the hydrosilylation smoothly to afford exclusively (E)ꢀvinylsilanes
in excellent yields and the hydrosilylation reaction of electronꢀdeficient substituted
ethyl propiolates also worked well to give the corresponding (E)ꢀαꢀethoxycarbonylꢀ
vinylsilanes in high yields with high regioꢀ and stereoselectivities. Furthermore, the
K2PtCl4/XphosꢀSO3Na/PEGꢀ400/H2O system can be recycled and reused eight times
without any loss of catalytic activity. This protocol will serve as an efficient, practical
and green way to prepare a variety of valuable functionalized βꢀ(E)ꢀvinylsilanes.
Experimental
Under an argon atmosphere, a mixture of K2PtCl4 (0.01 mmol), XphosꢀSO3Na (0.02
16

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mmol), PEGꢀ400 (0.67 mL) and H2O (0.33 mL) was stirred at 60 ºC for 30 min. Then,
terminal alkyne 1 (1.0 mmol) and hydrosilane 2 (1.5 mmol) were successively added
via syringe and the resulting mixture was stirred at 60 ºC for 6 h. After being cooled
to room temperature, the mixture was extracted three times with cyclohexane (3 × 5
mL). The combined cyclohexane phase was concentrated under reduced pressure, and
the residue was purified by flash column chromatography on silica gel (light petroꢀ
leum ether or light petroleum ether–ethyl acetate) to afford the desired product 3 or 4.
The residue of the extraction was heated to 60 ºC in vacuum for 20 min to remove
the residual cyclohexane, and then subjected to a second run of the hydrosilylation
reaction by charging with the same substrates (terminal alkyne 1 and hydrosilane 2)
under the same conditions without further addition of K2PtCl4 and XphosꢀSO3Na.
Acknowledgements
We thank the National Natural Science Foundation of China (No. 21462021) and Key
Laboratory of Functional Small Organic Molecule, Ministry of Education (No.
KLFSꢀKFꢀ201701) for financial support.
Notes and references
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Graphical contents entry
Recyclable and reusable K2PtCl4/Xphos-
SO3Na/PEG-400/H2O system for highly
regio- and stereoselective hydrosilylation
of terminal alkynes
Recyclable K2PtCl4/Xphos-SO3Na/PEG-400/H2O system has been
developed for highly regio- and stereoselective hydrosilylation of
terminal alkynes leading to β-(E)-vinylsilanes exclusively.
Caifeng Xu, Bin Huang, Tao Yan,
Mingzhong Cai*